Which issue(s) this PR fixes:
Fixes #1657

What this PR does / why we need it:
Seems that a full GC have to run in order to collect the wasted String object in @chunk.
However, full GC is not performed frequently.

This patch purges internal memory in wasted String object immediately.

Here is memory usage retrieved by ObjectSpace.memsize_of_all.

Reproduce

• Configuration

<source>
  @type tcp
  tag testing
  format json
  port 10130
</source>
<match **>
  @type forward
  require_ack_response true
  <server>
    host 127.0.0.1
    port 10131
  </server>

  ## Tune the buffer parameters
  <buffer tag>
    flush_mode interval
    flush_interval 1s
  </buffer>
</match>

• test data

require "socket"
require "json"

i = 0

Thread.new do
  s = TCPSocket.open("localhost", 10130)
  data = {"message": "a" * 512 * 1024}

  loop do
    puts "[#{i}]"
    data["number"] = i
    s.puts data.to_json
    i += 1
    sleep 0.01
  end

  s.close
end

sleep 20

Docs Changes:
Not needed.

Release Note:
buf_memory: improve memory performance